Apparatus for collection and removal of gases from an aluminum reduction cell

ABSTRACT

An apparatus for collection and removal of gases emitted by an aluminum reduction cell is formed within an anode superstructure having two sidewalls spaced from each other. At least one gas transmitting channel having varying cross-section extends along the superstructure toward an end thereof associated with a gas treatment system. A plurality of gas input sections are provided within the superstructure and formed by a pair of spaced from each other members extending transversely to said sidewalls. At least one nozzle is formed at an interface between the gas transmitting channel and the respected gas input section. The ratio of cross-section area of the nozzle at an outlet thereof to a cross-section area of the gas transmitting channel and the output of the nozzle decreases in the direction of the end of the gas transmitting channel associated with the gas treatment system.

FIELD OF THE INVENTION

This invention relates to non-ferrous metallurgy in general, more particularly to gas collection and removal technology for production of aluminum by electrolysis.

BACKGROUND OF THE INVENTION

Electrolytic cells are well known in the art of metallurgy and specifically, in the production of aluminum by the electrolysis of alumina. Electrolysis cells contain tanks open at the upper end thereof and having base formed by metal bars supporting blocks of carbon. The blocks of carbon are connected by a lining and act as a cathode. The tank contains an electrolysis bath consisting of alumina dissolved in cryolite which is heated to a temperature of about 950° C. to 1050° C. Anodes made of carbon are dipped into the cryolite bath.

Upon passing electric current through the cell, the alumina decomposes into aluminum and forms a metal bath which is deposited on the cathode. Oxygen gas is released during this process. The lower portion of the carbon anode which is dipped into the high temperature electrolysis bath. Due to the presence of combustible oxygen, the anode becomes burnt while the upper portion softens.

Combustion of the anode is accompanied by the substantial emission of fumes consisting of gases such as carbon dioxide, carbon monoxide, sulfur dioxide, gaseous hydrofluoric acid, and particles of carbon, alumina, and fluorine-containing compounds. A solid crust typically forms on the top of the electrolytic cells. The crust traps gas in the electrolytic cells forming a buildup of pressure. The crust often breaks to alleviate the pressure buildup and allow the buildup gases to escape.

The gas contains noxious compounds which are detrimental to the health of individuals and the environment. Therefore, the gas must be collected and purified. The increased volume of production of electrolytic cells is leading to an acute problem with regards to the gas collection and purification. Due to the increased size and volume of production facilities as well as stringent environmental regulations, costs of gas disposal are increasing rapidly. Further, inefficient removal of gases from electrolysis cells compromises the efficiency of aluminum production and often results in more gases being released into the workroom. Thus, the air of the building is more hazardous to breathe and extensive filters and purification techniques are needed before the air is ventilated to the atmosphere.

A typical method of collecting gases from the electrolytic cell involves collecting the gases from the roof of a building or contained space. The collection unit will collect gas emitted from many individual electrolytic cells. A large volume of air must be purified before exiting the facility, but the concentration of noxious fumes is generally low enough that it may be breathed, albeit uncomfortably. To purify the amount of air in a large facility requires a vast and expensive filtration system. The upper part of an electrolytic cell of type Hall-Heroult usually comprises an anode superstructure consisting of covers or an enclosure including a gas extractions system. Some auxiliary equipment may also be attached such as an anode beam with jacks, crust breakers and a system for raw material dosing inside the superstructure. The gas extraction system or, more precisely, the gas channels on the anode superstructure can be divided into two separate systems (right/left) which go out to a collection channel which is preferably arranged along the hall wall.

U.S. Pat. No. 1,473,718 discloses gas collection and removal apparatus having an inner space of the superstructure divided throughout its height into five collection zones by horizontal partitions. Each collection zone is extended throughout the length of the superstructure and is approximately equal to the width of two flaps covering the cell or two hood flaps. In this arrangement the space between two neighboring partitions forms predetermined channels adapted to remove gas from each collection zone. The channels converge into a common gas collecting main connected with central suction device. The volume of gases collected by suction by each channel is controlled by temperature sensors and rotary dampers (shutter) installed in the respective channel. The principal object of this prior art apparatus is to automatically control gas collection intensity during various operational modes of the cell.

Subdividing the gas duct channels of the superstructure of this patent by the partitions into a system of parallel channels having constant cross-section S1 substantially increases undesirable gas-dynamic resistance of the respective apparatus. In order to overcome such resistance considerable, low pressure zone or vacuum is required. Furthermore, the shutters installed in each channel produce additional gas-dynamic resistance resulted in the alumina deposits within the channels. High temperature, abrasive particles, fluorine compounds and magnetic fields either hamper or even substantially preclude proper operation of the shutters.

The alumina deposits within the channels redistribute flow of gases within the channels making impossible uniform removal of gases through the entire length of the cell.

Since the inner space of the superstructure of this prior art apparatus is subdivided by solid, horizontal partitions, installation and use of metering devices for automatic alumina feeding (AAF) is precluded. Another major drawback of this prior art arrangement is that it imposes substantial restrictions when used with the entire class of anode busbars having the side busbars connected between each other.

Patent RU No. 2,218,453 teaches an apparatus for collection and removal of gases from an aluminum reduction cell equipped with automated alumina feeding metering device. This apparatus consists of a superstructure formed with vertical walls, upper and lower stiffening members and gas duct channels provided with suction ports or windows adapted for collection and removal gases. The vertical walls of the superstructure contain double elements. In this manner, two gas channels are formed for collection and removal of gases from the top part of the superstructure. Limiting arrangements are installed at an angle in each gas channel to form a suction slot with constant width and variable height. The height of the gas channels increases toward the end of the superstructure connected with the gas treatment system.

In this prior art apparatus the gas jets enter the gas channel vertically at high speed to form vortex zones which cause increase in the gas-dynamic resistance to the gas flow of the device. The higher the rarefaction (vacuum) in the gas treatment system, the greater is the resistance to the gas flow. As a result, the rarefaction (vacuum) required for the removal of gases throughout the entire length of the cell has not been provided. In this apparatus the gas efficiently removed only from a portion of the cell. The resulted non-uniform removal of gases over the length of the cell creates stagnant zones causing undesirable penetration of the gas streams into the potroom.

In order to assure uniform gas collection over the entire length of the cell, only the cross-section of gas channel S1 was changed. The area of the suction slots S2 is maintained having constant width and is open along the entire length of the superstructure. In this instance, the incoming gas volume substantially exceeds the capacity of the outlet section of the gas channel. An alternate possibility of reducing the width of the suction slot is typically resulted in the slot being obliterated with alumina. The above-discussed prior art apparatus is not able to provide uniform removal of gases over the entire length of the cell without changing the area of suction slots S2 and respective configuration thereof.

SUMMARY OF THE INVENTION

One of the objects of the invention is to enhance efficiency of gas removal with one-sided gas collection from cells arranged side by side in the potroom.

Another object of the invention is to provide uniform removal of gas from the aluminum reduction cell over its entire length thereof and to reduce gas-dynamic resistance of the system, especially applicable for gas removal from aluminum reduction cells with one-sided gas collection arranged in side-by-side relationship in the potroom.

The apparatus of the invention for collection and removal of gases emitted by an aluminum reduction cell equipped with an automatic alumina feeding system consists of a superstructure with a substantially vertical double walls and gas channels of variable cross-section. The height of the gas channels increases toward the end of the superstructure connected with gas treatment system. Significantly, in the apparatus of the invention, between the substantially vertical walls of the superstructure, in the zone of operation of automatic alumina feeding system crust breakers and cell metal extraction devices, plates are provided forming inlet channels. The plates are equipped at the gas channel input with nozzles so as to assure the required speed and direction of gas flow movement. The ratio of cross-section area of the exit area of the nozzle to the cross-section area of the gas channel at the exit area of the nozzle is reduced toward the end of the cell connected to the gas treatment system.

Decrease of the ratio between areas S2/S1 along the length of the gas channel in the superstructure in the direction of the end of the cell connected with the gas treatment center makes it possible to distribute the gas-dynamic resistance. In this manner the required rarefaction (vacuum) is provided along the entire length of the gas channel. Such arrangement facilitates uniform removal of gases over the entire length of the cell.

In the device of the invention the inlet channels are provided in the zone action of alumina feeding system crust breaking and metal extraction equipment, i.e. in the zones where gases escape from under the alumina crust etc. Thus, the equipment is disposed under the hooding in the area most efficient for gas collection.

The use of nozzles provides for the required initial speed and the angle of entry of gas flows into the gas channel without formation of vortexes and zones of stagnation. Furthermore, the angles of nozzles position make facilitate distribution of incoming gas flows over the height of the gas channels. This considerably reduces the gas-dynamic resistance of the system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial view of an aluminum reduction cell illustrating elements of the apparatus for collection and removal of gases;

FIG. 2 is a partial view of a superstructure illustrating one embodiment of the gas input section connected to the gas transmitting channel of the superstructure and cell hooding;

FIG. 3 illustrates another embodiment of the gas input section connected to the gas transmitting channel;

FIG. 4 illustrates further embodiment of the gas input section connected to the gas transmitting channel; and

FIG. 5 is the chart illustrating estimated efficiency of the gas removal from electrolysis cell.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to drawings in general and FIG. 1 in particular which is a partial view of the apparatus for collection and removal of the emitted gases. More specifically, FIG. 1 shows a partial view of an anode superstructure associated with an enclosure or hooding 10 of electrolysis cell. The anode superstructure is formed by at least two sidewalls 2, so as to define an operation space 19 therebetween. At least one gas transmitting channel 3 extends longitudinally at a top area of the superstructure from a proximal end 12 to a distal end 14 having an output branch pipe which is directly or indirectly connected to the aluminum plant gas treatment system. The gas transmitting channel 3 is formed by at least a top wall 15 and a bottom wall 17 serving as an interface between the gas channel and the multiplicity of gas input sections. The cross-section area S1 of the gas transmitting channel 3 increases in the direction of the distal end 14. In this manner, the vertical extension E of the gas transmitting channel 3 is maximal at the distal end 14 near connection with the gas treatment system. Multiple pairs of spaced from each other panels 5 are provided within the anode superstructure, defining a multiplicity of vertically oriented gas input sections 6. The panels 5 are generally oriented transversely to the sidewalls 2 and extend substantially vertically within the operational space 19.

As best illustrated in FIGS. 2, 3 and 4 gas input sections 6 extend through the anode superstructure from an inlet 16 provided at the upper area of the hooding 7 to an outlet 18 disposed at the gas transmitting channel 3. Each gas input section 6 is formed with at least one nozzle 8 provided at the interface 17 between the respective gas input section and the gas transmitting channel. In the embodiment of FIG. 2 the gas input section 6 is formed with multiple nozzles 8 positioned at an angle to the interface 17. The cross-section of the nozzles S2 is being decreased in the direction of movement of the gas within the gas channel 3. This means that the smallest cross-section of the respective nozzle is at the distal end 14 at the output branch pipe connected to the gas treatment system.

An essential feature of the invention is that the ratio S2/S1 of the cross-section area S2 of the nozzles 8 at the outlet area thereof to the cross-section area S1 of the gas transmitting channel 3 situated at the location of the nozzle decreases in the direction of the distal end 14 associated with the gas treatment system.

The gas input sections are disposed in the operation zone of alumina feeding system crust breakers. The drawings show the phantom axis lines corresponding to the location of the alumina feeding system break rods. The bins 32 of the alumina feeding system are also shown.

Referring now to FIG. 2 illustrating one embodiment of the gas input section 6 which is provided within the anode superstructure between the closure or hooding 7 and the gas transmitting channel 3. In this embodiment substantial portions of the opposing panels 5 are substantially parallel each other. The nozzles 8 are provided within the interface 17 in such a manner that longitudinal exes thereof are disposed at an acute angle to the interface 17. Such orientation of the nozzles facilitates entry and movement of the gases within the channel 3.

In the embodiment of FIG. 3 the panels 5A are disposed at an angle to each other, so that the gas input section 6A is formed having a pyramidal configuration. Thus, the inlet area 16A of the gas input section has the cross-section substantially larger than the cross-section of the outlet 18A positioned at the interface 17A. In this embodiment, the nozzle 8A includes a buffer 22 which is adapted to further guide the exiting gas flow in the direction of the distal end 14. The gas input section 8A illustrated in FIG. 3 can be also formed having a frusto-conical configuration.

Referring now to FIG. 4 illustrating still another embodiment of the gas input section 6B. The spaced from each other panels 5B are positioned at an angle to each other and at an angle to the interface 17B between the gas transmitting channel and the respective gas input section. The longitudinal axis of the gas input section is also positioned at an angle to the interface. To facilitate smooth entry of the gas flow from the gas input section 6B into the gas transmitting channel 3B, the outlet areas of the panels 5B are formed having curvilinear sections 24B and 26B. A step-type formulation 28B is provided at an area of entry of the gas input section into the gas transmitting channel.

During operation of the electrolysis cell, the gases through breaks in the crust escape into the space under the hooding 7. Then, the convective gas flows through gas input sections 6 and nozzles 8 enter into the gas transmitting channels 3 and further delivered into the gas treatment system of the potroom (not shown in the drawing).

FIG. 5 represents a chart illustrating the results of the experiment and reflecting the calculated efficiency of the gas removal throughout the length of the electrolysis cell. The prior art gas removal systems used in the experiment had variable cross-section S1 of the gas transmitting channel only. However, in the device of the invention the ratio S2/S1 decreased in the direction of the distal end 14 of the gas transmitting channel connected to the potroom gas treatment system. In the device used in the experiment, the gas input sections of the invention were made according to the embodiment of FIG. 2. The calculations were carried out utilizing STAR-CD computation hydrodynamics program for the same cell design. The mathematical modeling of the gas removal process for the prior art device have demonstrated that the gas was removed only from about one-half of the entire cell length. It is apparent from the charts of FIG. 5 that upon decreasing the ratio S2/S1 in the device of the invention in the direction of the distal end 14 of the gas transmitting channel connected with the gas treatment system, the gas was removed uniformly throughout the entire length of the electrolysis cell. Certain increase in the volume of the removed gases at the proximal end location is necessary for the uniform gas removal during metal extraction from the cell.

The above discussed structure of the apparatus for collection and removal of gases of the invention discussed hereinabove provide the required speed and direction of gas flow within the gas transmitting channel. Although, the invention has been described with reference to specific embodiments illustrated in the figures of the application, it should be understood that further obvious modifications are within the scope of the invention. For example, the anode superstructure with at least one gas transmitting channel has been discussed hereinabove. However, it should be understood that the embodiment of the invention with multiple gas transmitting channels and multiple rows of the gas input section associated therewith are within the scope of the invention. 

1. An apparatus for collection and removal of gases emitted by an aluminum reduction cell provided with an alumina feeding system, said apparatus comprising; an anode superstructure, said anode superstructure formed by at least two spaced from each other side walls to define an operation space therebetween, at least one gas transmitting channel having varying cross-section extending along said superstructure, a distal end of said at least one gas transmitting channel being associated with a gas treatment system, height of said at least one gas transmitting channel increasing toward said distal end thereof, a plurality of gas input sections within said superstructure, each said gas input section formed by a pair of spaced from each other members extending transversely to said sidewalls, each said gas input section extending through said superstructure from an inlet to an outlet provided at said gas transmitting channel and adapted for delivery of said gases to said at least one gas channel, at least one nozzle formed at an interface between each said gas input section and said at least one gas transmitting channel, so as to provide required speed and direction of a gas flow within said at least one gas transmitting channel.
 2. An apparatus according to claim 1, wherein ratio of an outlet cross-section area of said nozzle to a cross-section area of the gas transmitting channel at a location of said nozzle outlet decreases in the direction of said distal end of said at least one gas transmitting channel.
 3. An apparatus according to claim 2, wherein said device is adapted for collection and removal gases emitted by aluminum reduction cell having an automated alumina feeding system.
 4. An apparatus according to claim 1, wherein said plurality of gas input sections is formed within an operation zone of said alumina feeding system.
 5. An apparatus according to claim 4, wherein said gas input sections are provided at the location of breakers and metal extraction devices of said aluminum reduction cell.
 6. An apparatus according to claim 1, wherein said spaced from each other members are substantially parallel to each other and are connected to said interface, said nozzles extend through said interface.
 7. An apparatus according to claim 6, wherein said interface is a wall longitudinally extending between said plurality of gas input sections and said at least one gas transmitting channel, a longitudinal axis of each said nozzle is positioned at an acute angle to said longitudinally extending wall.
 8. An apparatus according to claim 2, wherein said members forming said gas input sections are positioned at an angle to each other, so that each said gas input section is formed having a pyramidal configuration, wherein at inlet area of each said gas input section has substantially larger cross-section than the cross-section at the outlet area thereof provided at said interface.
 9. An apparatus according to claim 8, wherein each said nozzle is provided at said outlet of the respective gas input section, said nozzle being formed with a buffer so as to further direct the gas flow exiting said nozzle into the gas transmitting channel in the direction of said distal end associated with the gas treatment system.
 10. A device according to claim 8, wherein each said gas input section is formed having a frusto-conical configuration.
 11. An apparatus according to claim 2, wherein said spaced from each other members forming the respective gas input section are oppositioned at an angle to each other and to said interface, wherein a longitudinal axis of each said gas input section is positioned at an angle to said interface.
 12. An apparatus according to claim 11, wherein a curvilinear connection area is provided between each said member and said interface to facilitate smooth entry of said gas from said gas input section into said gas transmitting channel. 